The present invention relates to a power supply for a device that comprises a coil, and a drive circuit coupled to the coil. The invention also relates to a display apparatus comprising such a power supply.
In known configurations, the power supply provides a preferably stable, constant power supply voltage. The known configurations have for a disadvantage, that the power losses are considerable, especially in the part of a scan of the picture display tube, wherein the current to be output to the deflection coil has an absolute value, which is lower than a maximum value.
It is, inter alia, an object of the invention to provide an improved power supply. To this end, the invention provides a power supply as defined in the independent claims. Advantageous embodiments are defined in the dependent claims.
A preferred embodiment of the present invention provides a power supply for a vertical deflection circuit that comprises a deflection coil for connection to a picture tube, a drive circuit connected to the deflection coil, and a deflection controller for controlling the drive circuit, in which the power supply comprises a first input connected to an output of the drive circuit and one side of the deflection coil, a second input connected to a power source, a power supply output connected to the drive circuit, and an adapter for adapting the power supply to the output from the second input dependent on the first input.
Therefore the power supply according to the present invention is based on the concept of monitoring the voltage at the output of the drive circuit. The power supply is adapted by the adapter, dependent on the thus obtained information. Therefore, the power loss in the vertical output circuit is, according to the present invention, minimized, when the above mentioned voltage level is regulated to a level, where the drive circuit at the output thereof shows a linear behavior. Thereby, the supply voltage is as low as necessary for a good operation. Preferably, the supply voltage follows the voltage over the deflection coil, but is minimally equal to the minimum voltage required by the vertical deflection output circuit IC.
The drive circuit can comprise a bridge circuit having two outputs to the deflection coil and a return or feedback input, and where the power supply comprises two first inputs, each connected to a side of output stages to the deflection coil. In such an embodiment, for the first half of a scan, the voltage between the first of the two outputs and ground is monitored, whereas for the second part of a scan, the voltage between the second of the two outputs and ground is observed. The values of the voltages are optimized by adjusting in accordance with the requirements of the deflection coil at a certain point in time of a scan.
Modulating the supply voltage can be done in at least two ways. It can be linear, where a power transistor is used as a variable resistor in series with the vertical deflection output stage. Power consumption is then transferred from the output stage to the modulating power supply. A better way is to use a switch mode power supply. Here the pass transistor is used as a switch. The output can be filtered by an inductor and a capacitor. In this case the total power consumption is greatly reduced. The system needs an inductor and a capacitor that cannot be integrated. The supply voltage of the vertical deflection stage may contain more ripple, which may be visible on the screen if the vertical deflection stage is not good in rejecting the ripple in the power supply. This modulation of the supply may have some effect on the stability of the deflection amplifier. The power supply (ripple) rejection ratio of the amplifier should be high enough not to create visible artifacts on the screen.
An advantage of the invention is that the customer, i.e. the television or monitor set maker, does not have to worry about choosing the right supply voltage for the vertical deflection output stage. The supply modulation circuit will take care that the supply voltage of the output stage is always optimal. Compared to the situation in which the supply voltage is higher than optimal, the power saving can be significant. Another advantage is that the heat sink can be reduced in size.
Further features, embodiments and advantages of the present invention will be described here below in more detail, while referring to the accompanying drawings.